Temperature dependence of half flux quantum in YBa2Cu3O7-y tricrystal thin film observed by scanning SQUID microscopy

One of the most important topics in high critical temperature superconductors is the symmetry of order parameter. The most powerful and direct method to show the evidence of d-wave symmetry is the measurement of the half flux quantum at the tricrystal boundary of a superconducting thin film. We present the observation of the half flux quantum in a YBa2Cu3O7-y thin film on a SrTiO3 tricrystal substrate by using a scanning SQUID microscope (SSM). The SSM observation is done from 3 to 90 K. The vortex at the tricrystal point corresponds to Phi(0)/2 (= 1.03 x 10(-15) Wb) and this half vortex is observable up to near the critical temperature. The results indicate the evidence of d(x2-y2) symmetry of the superconducting order parameter. Josephson vortices at grain boundaries are also observable and some of them are mobile with increasing the temperature. (C) 2002 Elsevier Science B.V. All rights reserved.